According to existing technology, a user's focus point can be determined from a picture of the eye, wherein a semi-reflective mirror positioned at an approximately 45 degree tilt angle is used for a near-right-angle reflective optical path that connects the eye and the camera (patent document 3). The same technique applies to HMDs and other similar devices mounted on a user's head (patent document 1).
Technology for changing the video on a video display of a HMD by means of neck movement is disclosed in patent document 2.